AIR CIRCUIT BREAKER 101: A COMPREHENSIVE GUIDE

Air Circuit Breaker (ACB)

Air circuit breaker is the simplest form of circuit breaker. In this circuit breaker, the interruption of the arc is mainly based on the natural deionization of gases by a cooling action and lengthening of the arc.

Working Principle

The working principle of the air circuit breaker is to prevent the re-formation of the arc after current zero has been achieved by creating a gap between the contacts that can withstand the system recovery voltage.

In an air circuit breaker, the cooling of the arc creates a high voltage gradient, which is achieved by directing the arc to an arc chute of the large contact area of insulating material. The walls of the arc chute are designed in such a way that the arc not only is directed but also driven into the serpentine-like channel of the arc chute. The walls of the arc chute of the air circuit breaker can sufficiently achieve the required cooling. High-temperature plastics with reinforced glass fiber and ceramics are used to make the arc chute.

The lengthening of the arc in the chamber increases the voltage drop per unit length which makes high arc voltage and arc resistance. This affects the power factor in a way that the instantaneous supply voltage as the current approaches zero is much below the peak voltage value. The high value of the arc resistance helps to dampen the transient recovery voltage oscillations, making amplitude factor unity.

Construction

The following are the main parts of the air circuit breaker: –

Main Contacts: These contacts carry rated continuous current and consist of moving contact and fixed contact.

Moving Contact Assembly: The assembly has solid copper spring-loaded rollers ringed with silver. The rollers freely roll a few degrees, each time the contact is operated. The assembly consists of supporting insulators, the operating rod that connects to the operating mechanism, spring mechanism for very fast separation during the fault.

The design of the moving assembly is such that it guides the arc formed during the separation of the contacts to the arc chute for efficient extinguishment.

Fixed Contact Assembly: They are two levelled copper contact bars, with silver pads at contact faces. The rollers bridge these two fixed contacts on opposite poles. This arrangement provides for durability and minimum temperature rise.

Arcing Contacts: These are the contacts, which face the effects of the arc. During the closing operation, it is arcing contacts that close first, and during tripping, it opens at the last.

Arc Chute: All arc chutes of air circuit breakers are made of an insulating arc-resisting material and surround each pole. The size of the chute depends upon the number of arcing contacts.  At the top inner surface, the arc chute is fixed with steel plates. The purpose of these steel plates is to increase the rate of upward rise of the arc into the chute by magnetic action. It also splits the arc and assists in cooling the arc.

Functions of arc chute of air circuit breaker

• It confines the arc within a restricted space.
• It provides mechanical protection for the personnel for any external object.
• It provides rapid cooling of the gases to ensure extinction by de-ionisation.

Operating Mechanism: Mechanisms used in air circuit breakers are designed either for manual operation or electrical or pneumatic operations. This mechanism takes care of trip-free operation and opening and closing of contacts. The operating mechanism also provides a lockout feature that prevents closing of the air circuit breaker, while any work is being carried out.

Arc Runners or Arcing Horns: As the arc leaves the vicinity of the contacts of the air circuit breaker, it commutes to a pair of run-out horns. Simultaneously the blowout coil is energized. In doing so outer blowout system is switched on. This blow-out coil provides a magnetic field, which causes the arc to travel upward thereby increasing the length. As the length of the arc is increased at a particular stage the system voltage is unable to sustain the arc and the arc gets extinguished. 

Types of Air circuit breaker

There are various types of air circuit breakers.

Plain Break Type

In this type of Air Circuit Breaker, the contacts are made in the shape of two horns. The arc initially strikes across the shortest distance between the horns, and it is then driven steadily upwards. As the contacts go on separating the gap between them increases and the arc also follows the contacts. When the horns are fully separated the arc extends from one tip to the other resulting in arc lengthening and cooling, thus extinguishing the arc. The relative slowness of the process and the possibility of the arc spreading to adjacent metalwork limits the application of this type of air circuit breaker to about 500 volts and in low-power circuits only.

Magnetic Blow-out Type

These types of air circuit breakers are used up to 11 kV voltage level. Here, the extinction of the arc is carried out by means of a magnetic blast. To achieve this, the arc is subjected to the action of a magnetic field set up by coils connected in series with the circuit being interrupted. Such coils are called blow-out coils because they help in the arc being magnetically blown out.

The arc is blown magnetically into arc chutes where the arc gets lengthened, cooled, and extinguished. The arc shield prevents the spreading of the arc to adjacent metalwork. Since the breaking action is more effective with high current, the breaking capacity of these air circuit breakers is therefore high.

Arc Splitter Type

In this type of air circuit breaker, the blowout consists of steel inserts in the arcing chutes. The steel inserts are arranged in a way that the magnetic field induced by them, causes the current in the arc to move upwards still faster. The arc is divided into a number of short arcs in series by the steel plates.

The voltage distribution along the length of the arc across the steel plates is not linear but is accompanied by large voltage drops. This voltage drop automatically helps for quick extinction of the arc. The arc gets rapidly and effectively cooled when it comes in contact with a relatively cool surface area of the steel plates. The movement of the arc can be natural or assisted by a magnetic blow out. These air circuit breakers generally consist of three single pole units linked together by an insulated crossbar.

Maintenance of Air Circuit Breaker

Before any commencement of work, it is important to make sure that all power-closing devices of air circuit breakers are made inoperative. This can be achieved by: –

• For solenoid-closed air circuit breakers, the solenoid supply should be isolated.
• For spring-closed circuit breakers, the spring should be discharged, and in the case of a motor-operated spring closing mechanism, the supply to the motor should be isolated.
• Fuses and links in the control circuit and auxiliary circuit of the air circuit breaker should be withdrawn carefully.

Diagnostic Testing

Diagnostic testing of the air circuit breaker gives a ready indication of the state of the equipment and by comparison to the previous records it provides ample information on the condition of the equipment. The following diagnostic testing techniques are carried out wherever possible: –

Timings and travel tests and measurement of minimum operating voltages required for closing and tripping coils. The values so obtained will indicate the state of the operating mechanism.

Milli-volt drop tests or resistance tests between the terminals or across individual series connected components will give the condition of the components of the electrical equipment.

Opening Device (Trip)

Immediately prior to maintenance work commencement, the circuit breaker should be opened via an electrically operated trip coil or via the manual operation of the trip plunger.

Insulation

All the insulated portions must be cleaned and inspected. Porcelain and molded insulation should be inspected for cracks or other defects. Bonded and laminated fibrous insulation should be inspected for signs of cracking, blistering, or de-laminating. Insulation resistance tests are strongly recommended as the insulation gives such an indication.

Secondary wiring and fuses

All electrical connections must be tight so that good contact is maintained. The terminal boxes must be free from dirt and moisture. Verification of insulation resistance and continuity of wiring to the fuses, instrument transformers, motor, relays, and associated items must be done. All contacts including plug and socket contacts should be cleaned and re-lubricated with approved lubricants. Fuses should be tested for continuity and inspected for signs of damage or deterioration. Fixed contacts carrying the fuses should be cleaned and tested for satisfactory electrical contact.

Operating Mechanism

During the maintenance of the mechanism, care should be taken to avoid the fingers being trapped and the possibility of anybody being struck by moving parts. Clean and examine the mechanism. Replace the worn-out parts. It is particularly important to make sure that rolling or sliding surfaces in the trip mechanism are free from dried-up lubricant. The mechanical details of the closing mechanism should be checked and re-lubricated sparingly. They should be adjusted as required and checked for correct operation. The release oil plunger should be checked for freedom of movement but not lubricated. Extreme care should be taken to verify that the adjustment conforms to the manufacturer’s guide.

Auxiliary contacts, Linkages, and Indicating Devices

Examine the contacts and clean or replace them if necessary. verify good contact force and freedom of operating links. Ensure the correct timings of the contacts in relation to the circuit breaker contacts. Indicating devices such as ON/OFF indicators are inspected to ensure that they are in good condition and correct configuration.

Main and arcing contacts

Inspection of the main and arcing contacts is necessary for burning or other damages. Re-condition or replace them as required. Ensure that the jacking springs are exerting the correct force and the contacts are in good alignment. Normally the arcing contacts will show signs of burning and pitting and this is not a harmful sign if it is not excessive. The security of any arc-resisting tips should be checked. Hinge contacts should be examined for any signs of overheating, burning, welding, or other damages. They should be reconditioned, adjusted, or replaced as required. If such a state is found the cause should be investigated.

Slight discoloration or burning of copper or copper alloy contact is not necessarily harmful. If found may be removed by using a fine file or fine glass paper. In no circumstances, emery or sandpaper should be used. When cleaning the contacts, it is essential to ensure that the minimum amount of material is removed or else excessive filing of contacts may result in more rapid wear. Silver-plated contacts seldom require cleaning despite their black appearance. If any cleaning is required, a silver polish may be used. Abrasive materials should not be used on these contacts. When contacts are refitted or replaced, contact force, alignment, and wiper should be verified.

The flexible braid should be inspected, especially for fraying at the terminations, and replaced if necessary. When exposed to the atmosphere, the braids should be treated with a suitable protective compound, which would not impair their flexibility.

Contact pressure

During overhauling, maintaining contact pressure is of utmost importance. Apart from the pressure, the contour of the contact is also an important factor, since there are many possibilities of spoiling the contours. As such these points must be borne in mind while carrying out the maintenance. The amount of spring pressure may vary from one manufacturer to another. Hence it is recommended to follow the manufacturer’s instructions. Typically MV circuit breaker contact pressure will be in the order of 7 kg to 9 kg. Using a spring balance one can check this pressure. When the breaker is closed all three sets of phase contact should close simultaneously.

This can be checked again by inserting the paper between the surfaces or by a foolproof method of electrical bulbs. For this one board can be locally fabricated and a source of supply can be made available from the storage battery. Connect the battery in series with the contacts. (3 parallel circuit) when the contacts close all the bulbs will glow in synchronism. This is conveniently checked by closing the breaker slowly until the contact is made, when the contacts are set correctly the lights will glow together. If not the contacts may be adjusted accordingly so as to get all the lights to glow in synchronism.

Arc Chutes, Arcing Horns, and Inter-pole Barriers

Arc chutes and arcing horns should be inspected and cleaned thoroughly. If they are burnt they must be replaced. Arc control devices made from compressed fibrous materials, which cannot be cleaned without abrasion, should be replaced with new ones. Arc Chutes and additional coolers if fitted, should be cleaned. They should be inspected for loose or badly eroded splitter plates, which should be adjusted or renewed as necessary.

Inter-pole barriers should be inspected for the security of fixing and for any damage. They should be cleaned as necessary. As far as possible arcing tips and its control devices should be inspected to ensure that they are securely fastened, and adjusted in accordance with manufacturer instructions. Ensure that they have a good electrical connection. It should be remembered that the normal duty is to carry the arc root but if excessive burning or erosion is present, they should be replaced.

General Schedule of Inspection and Maintenance of ACB

FAQ’s